This invention relates to fuel cell systems and, in particular, fuel cell hybrid power plant systems and methods for gas distribution systems.
In conventional gas distribution systems, the natural gas being supplied by utilities such as, for example, Enbridge, Inc., an assignee of the subject application, is carried over extra-high pressure transmission and distribution pipelines. This extra-high pressure gas is reduced in pressure for distribution at a lower pressure which is typically 50-80 psig from an upstream pressure which is usually in the range of two to twenty times higher than the downstream pressure. These extra-high pressure pipelines deliver gas to city gate stations, or within urban centers, to district stations, which reduce the gas pressure so that the gas can then be distributed at lower pressures to natural gas users or consumers. The city gate or district stations are usually referred to as “pressure let down stations,” or “pressure reducing stations” and they must provide the necessary reduction of the extra-high pressure gas to the desired lower pressures.
The reduction in gas pressure is typically accomplished at each pressure let down station through pressure reducing valves. Accompanying the reduction in pressure is a refrigerant effect attributable to constant enthalpy expansion. This effect is similar to the cooling experienced when any gaseous compound (propane, compressed air, etc.) experiences a combination of significant pressure reduction and high volumetric flows. A physical manifestation of this can be noticed with the operation of a propane barbeque, where propane under pressure in the storage cylinder experiences a pressure reduction when the gas exits the cylinder. Under high flow conditions this refrigerant effect produces a cold exterior on the storage cylinder, which under extreme conditions can result in frost accumulating on the cylinder.
As above-stated, the same chilling or refrigerant effect occurs on natural gas pipelines where large gas flows undergo significant pressure drops. This cooling effect on gas pipelines is undesirable as it can cause heavy frost formation which can negatively affect pipeline system integrity and/or create movement of pavement near any pipelines which are within municipal road allowances. This significant cooling can also create control problems with hydrates (moisture) in the conveyed gas or fuel. To eliminate these concerns, gas utilities typically pre-heat the extra-high pressure gas before it is delivered to the pressure let down stations. This is usually accomplished by passing the gas through a pre-heater or heat exchanger having a thermal heat transfer fluid loop (typically a glycol loop) which is supplied heat via natural gas boilers. The heated heat fluid of the heat transfer fluid loop of the pre-heater heats the extra-high pressure gas sufficiently so that as its pressure is reduced in the let down station, the temperature of the gas is maintained above freezing, i.e., above 32° F. or 0° C.
As can be appreciated, the need to heat the ultra-high natural gas before delivery to the let down stations requires considerable energy and, therefore, reduces the overall efficiency of the gas distribution system. Also, the reduction in the gas pressure at the let down stations generates significant energy which to date has been untapped and wasted.
It is, therefore, an object of the present invention to provide a system and method for use in gas distributions systems which aims to provide an efficient and cost effective approach to reducing the pressure of gas supplied from extra-high pressure gas distribution/transmission lines.
It is also an object of the present invention to provide a system and method of the above type in which the energy generated in reducing the gas pressure is utilized to enhance system efficiencies.
It is also an object of the present invention to provide a system and method of the above type which utilizes components which contribute lesser amounts of contaminants to the atmosphere.